The World Health Organization (WHO) estimates 1.5 million new cases of breast cancer worldwide in 2010 with mortality exceeding 500,000, thus making breast cancer the leading cause for women deaths worldwide. The direct and indirect costs of breast cancer are staggering, with treatment costs approaching $8 billion annually and total costs, including treatment and losses in economic productivity amounting to $26 billion in 1996 (the latest year for which such information is available). According to Cancer Research UK (February 2011), a woman's risk of developing breast cancer has risen to one in eight over the last 10 years.
The most common imaging methods for cancer screening are Mammography and Magnetic Resonance Imaging (MRI). Other less frequently used methods include ultrasounds, positron emission tomography or thermology.
Currently, x-ray mammography is by far the incumbent technology for breast cancer screening. The process includes sandwiching the female breast between two plates, while low-energy ionizing X-rays are transmitted through the breast and imaged on a photographic plate. Women within the ‘at-risk’ age groups (between 49 and 70 years) are recommended to have regular screenings to enable early cancer detection. However a large number of patients are reluctant to undergo mammogram resulting in diagnosis at advanced stages of the disease. Detecting breast cancer at the advanced stages significantly reduces the effectiveness of medical intervention and life expectancy. Additionally delays lead to increased costs for both national and private health-service providers addressing breast cancer treatment.
The reluctance to avoid routine screenings has been identified to stem from the following issues with mammogram:    1) The use of ionising x-rays which pose a threat of causing (and increasing) cancer to patients in the course of repeated screenings leading to excessive exposure. The risk of radiation is higher among younger women due to their having higher breast tissue density, thus making current x-ray mammography devices potentially risky to this class of patients.    2) Negative user experience with existing equipment. The breast is pressed firmly between two plates, in order to obtain clear images from the X-ray. This process has been identified as a source of significant discomfort and pain to patients undertaking the procedure.
Research also shows there are significant risks of false diagnosis due to current the mammogram technique. There are two types of misdiagnosis:    1) False positive—this occurs when radiologists decide mammograms show abnormalities but no cancer is actually present. Natural News (2005) claimed that 70-80% of all positive mammograms do not, upon biopsy, show any presence of cancer. False-positive mammogram results can lead to anxiety and other forms of psychological distress in affected women. The additional testing required to rule out cancer can also be costly and time consuming and cause physical discomfort. Research (Brewer N T, et at 2007) shows that false-positive mammograms can significantly affect women's well-being and behaviour.    2) False negative—this occurs when mammograms appear normal even though breast cancer is present. According to National Cancer Institute (2010), screening mammograms miss up to 20 percent of breast cancers that are present at the time of screening.
Thermology derives diagnostic indications from detailed and sensitive infrared images of the human body. For breast cancer application, thermology has been primarily used for pre-cancer warning, but lacks credibility. The adoption rate for thermology was very slow and has only been deployed in a very limited number of clinics/health centres.
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. The use of ultrasound technology in breast cancer screening is in an early stage and subject to further development.
In Magnetic Resonance Imaging (MRI), a breast is scanned in an MRI device before and after the intravascular injection of a contrast agent (Gadolinium DTPA). The pre-contrast images are “subtracted” from the post-contrast images, and any areas that have increased blood flow are seen as bright spots on a dark background. Since breast cancers generally have an increased blood supply, the contrast agent causes these lesions to “light up” on the images. The methods provides excellent resolution (around 1 mm) but it much more expensive and complicated to use compared to e.g. mammography.
Microwave medical imaging (MWI) is an emerging alternative to well established X-ray techniques as demonstrated by a growing body of research (see, for example, L. E. Larsen and J. H. Jacobi, Eds., Medical Applications of Microwave Imaging. Piscataway, N.J.: IEEE Press, 1986). For example, recent research has shown its potential as a low-cost way of detecting cancer, offering increased patient acceptability without the potential hazards of ionising radiation (see, for example, S. Semenov, “Microwave tomography: Review of the progress towards clinical applications,” Phil. Trans. R. Soc. A, vol. 367, no. 1900, pp. 3021-3042, August 2009). In addition, non-invasive microwave-induced thermal therapy (hyperthermia) has also enjoyed significant progress over the last decade. Hyperthermia (40-45° C., typically for 60-90 min) causes direct cytotoxicity due to heat and increases sensitivity of cancer cells to radiation therapy and chemotherapy. As with most physics-based medical imaging and treatment methods, the clinical application and success of microwave-based techniques depends upon their ability to produce high-resolution imaging/focusing systems.
Microwave imaging operates via the principle of inverse scattering. An array of antennas is placed around the patient, focused towards the region of interest in the patient's human body. Due to the high permittivity and conductivity values of the tumour cells (which are typically much higher compared to the electromagnetic properties of the surrounding tissue), a significant portion of the wave energy will be scattered. This scattered energy is recorded and based on those signals the 3D image of the cancer cell region is reconstructed.
The imaging resolution of microwave imaging systems is compromised by the well-known Abbe-Rayleigh diffraction limit, which relates resolution to the illuminating wavelength of the imaging system. This is a critical issue for microwave systems, where typical wavelengths are of the order of centimetres. The imaging resolution is deteriorated by the reflection of the incident microwave radiation from the human tissue before reaching the tumour, and especially the skin. The microwave radiation has to transmit from outside the patient (e.g. air) through the skin and into the body. Since the skin has a relative dielectric permittivity around 50, a significant fraction of the incident energy is reflected and cannot by utilized to image the cancer regions. In electromagnetic terms, there is a large impedance mismatch between the air and the human body.
The present disclosure addresses some of these limitations. In particular, the present disclosure relates to improving the coupling of imaging radiation into a target medium.